In production of semi-conductor integrated circuits, magnetic bubble memory elements, etc., photoetching or lithography using energy rays, such as light (visible rays, ultraviolet rays, far ultraviolet rays), electron beams, ion beams, and X-rays is used. Resists used as a protecting film against etching in lithography- include a negatively working type and a positively working type, and the use of positive resists has recently been extending because they favor formation of finer patterns as compared with negative ones. Positive photoresists generally comprise a combination of a cresol novolak resin and a quinone diazide compound, which is insoluble in alkalis but is rendered alkali-solubilized on exposure to light.
There is a strong and increasing demand for resists for the formation of integrated circuits to have improved performance properties such as sensitivity, resolving power, heat resistance, and resistance to dry etching, with the tendency to high level of integration. In order to meet the demand, alkali-soluble novolak resins prepared from various phenol compounds have been proposed. For example, JP-A-60-176034 and JP-A-61-275748 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") teach to use a novolak resin prepared by condensation of m-cresol and xylenol with formaldehyde.
To increase resolving power, a resin is required to have a narrow molecular weight distribution and a low degree of dispersion so as to be sharply dissolved with a developer. To improve heat resistance and resistance to dry etching, it is effective to increase a molecular weight of the resin.
However, novolak resins mainly comprising m-cresol which have been made frequent use of as a resist base are apt to have a broad molecular weight distribution and a high degree of dispersion due to very high reactivity of m-cresol. Moreover, they have a limit of molecular weight because an increase in molecular weight in an attempt to improve heat resistance tends to be accompanied by gelation. Copolymers of m-cresol and xylenol have the same problems. Thus, sufficient improvements on resolving power, heat resistance and resistance to dry etching have not yet been reached.